I've never used them but will probably get some this fall. There are a couple of types of clips; one type consists of a base u-channel (aluminum or plastic) and a top piece that snaps into it. The second type is a bottom channel with a long serpentine spring that clips into it. The second kind is in this farmtek catalog (top of the page - aluminum poly latch). I've never ordered from them but other forum members seem to like them although their online catalog is terrible.

Here is a link that might be useful: Farmtek catalog

And another one here..

Here is a link that might be useful: wigglewire